Motorboating (electronics)

In electronics, motorboating is low frequency parasitic oscillation (unwanted cyclic variation of the output voltage) that occurs in audio and radio equipment and often manifests itself as a sound similar to an idling motorboat engine, a "put-put-put", in audio output from speakers or earphones. It is a problem encountered particularly in radio transceivers and older vacuum tube audio systems, guitar amplifiers, PA systems and is caused by some type of unwanted feedback in the circuit. The amplifying devices in audio and radio equipment are vulnerable to a variety of feedback problems, which can cause distinctive noise in the output. The term motorboating is applied to oscillations whose frequency is below the range of hearing, from 1 to 10 hertz, so the individual oscillations are heard as pulses. Sometimes the oscillations can even be seen visually as the woofer cones in speakers slowly moving in and out.

Besides sounding annoying, motorboating can cause clipping of the audio output waveform, and thus distortion in the output.

Although low frequency parasitic oscillations in audio equipment may be due to a range of causes, there are a few types of equipment in which it is frequently seen:

As with all electronic oscillation, motorboating occurs when some of the output energy from an amplifying device like a transistor or vacuum tube gets coupled back into the input circuit of the device (or possibly into an earlier stage of the amplifier circuit) with the proper phase for positive feedback. This indicates there is an unwanted feedback path through the circuit from output to input of an amplifying stage. The technical conditions for oscillation, given by the Barkhausen stability criterion, are that the total gain around the feedback loop (comprising the amplifying device and the feedback path) at the oscillation frequency must be one (0 dB), and that the phase shift must be a multiple of 360° (2π radians). Since most amplifying devices, transistors and tubes, are inverting, with the output signal 180° opposite in phase from the input, the feedback path must contribute the other 180° of shift.

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